1. Field of the Invention
The present invention relates to a surface acoustic wave device such as surface acoustic wave (SAW) filter, surface acoustic wave resonator and surface acoustic wave guide, and more particularly relates to a surface acoustic device having an electrode structure which can realize a natural single-phase unidirectional transducer property.
2. Description of the Related Art
Heretofore, the surface acoustic wave device has been desired to have large electromechanical coupling coefficient K.sup.2, small power flow angle PFA, small reflection by electrodes due to elastic perturbation, small temperature coefficient of delay time TCD and so no. Furthermore, recent electronic devices have been required to be small in size, light in weight and economical in power.
There has been widely used a transversal type surface acoustic wave device including a substrate having a piezoelectric property, a transmitter side transducer having interdigitally arranged positive and negative electrodes which are connected to two opposite phase output terminals of a single-phase signal source and a receiver side transducer having interdigitally arranged positive and negative electrodes. Such a transversal type surface acoustic wave filter has been used for extracting a signal component within a given frequency range.
In such a surface acoustic wave filter, it has been required to reduce an insertion loss and to suppress undesired ripple connects within a given frequency range. Since usual electrode structures have a bidirectional property, the insertion loss could not be reduced smaller than a theoretical insertion loss of 6 dB. This insertion loss of 6 dB is too large to realize a SAW filter having a desired property.
In order to mitigate such a drawback, there has been proposed a multi-transducer system, in which on both sides of each of a plurality of transmitter side transducers there are arranged a pair of receiver side transducers. In a surface acoustic wave filter of such a multi-transducer system, it is possible to reduce the insertion loss to about 1.5-2.0 dB. However, in the multi-transducer system, the control of a plurality of transducers is very difficult and good phase property and frequency property could not be attained. Moreover, it is quite difficult to manufacture such a multi-transducer system device. In order to improve the faculty of the surface acoustic wave device, it is also very important to flatten a phase characteristic and to suppress pass band ripples and suppression band.
It has been proposed to use a unidirectional transducer which can realize a very small insertion loss which is theoretically lower than 1 dB and has excellent phase and frequency characteristics. There have been proposed various types of the unidirectional transducers. They may be roughly classified into (a) multi-phase type unidirectional transducer, and (b) single-phase type unidirectional transducer. As the latter single-phase type unidirectional transducer, there have been proposed various types. For instance, there have been proposed a single-phase unidirectional transducer utilizing an asymmetry of electrode structure and an internal reflection due to the mass load effect, a reflection bank type unidirectional transducer having a reflection bank between exiting electrodes, a single-phase unidirectional transducer utilizing a reflection by floating electrodes, and a natural single-phase unidirectional transducer utilizing anisotropic property of a substrate. In surface acoustic wave devices including these unidirectional transducers, the directionality is realized by making exciting wave and reflected wave to be in-phase in a wave propagating direction and to be opposite phase in an opposite direction.
Heretofore, substrates of surface acoustic wave devices are made of single crystals of quartz, lithium tantalate (LiTaO.sub.3) and lithium niobate (LiNbO.sub.3). In U.S. Pat. No. 5,698,927, there has been proposed to use a lithium tetraborate (LiB.sub.4 O.sub.7) single crystal as a substrate of the natural single-phase unidirectional transducer. However, the known single crystals are not optimal for the surface acoustic wave device and it is difficult to obtain a surface acoustic wave filter having desired characteristics.
For instance, in order to realize a surface acoustic wave resonator, it is required that the electro-mechanical coupling coefficient K.sup.2 is large, the reflection is large, the power flow angle PFA is small and the temperature dependency TCD is small. However, the electro-mechanical coupling coefficient K.sup.2 of quartz is about 0.2 and the reflection of quartz is also small. Therefore, it is rather difficult to realize a surface acoustic wave resonator having good characteristics by using the quartz.
A substrate for the surface acoustic wave filter is required to have a large electro-mechanical coupling coefficient, a small reflection, a small power flow angle and a small temperature dependency. However, a piezo-electric substrate having such characteristics has not been proposed.
In the above mentioned known single-phase unidirectional transducer except for the natural single-phase unidirectional transducer, the electrode structure has become very complicated. Particularly, a distance between adjacent electrode edges and a width of an electrode have to be smaller than .lambda./4. Upon increasing an operation frequency, said distance and width become very small, and it is quite difficult to manufacture electrodes having small distance and width.
In order to mitigate the above mentioned drawbacks, there has been proposed a natural single-phase unidirectional transducer (NSPUDT), in which the unidirectionality is realized by utilizing the anisotropy of the piezoelectric substrate although the normal type electrode structure having an electrode edge distance of .lambda./4 and an electrode width of .lambda./4. In the surface acoustic wave device using the NSPUDT operation, as the piezoelectric substrate showing the anisotropy there have been known quartz substrate, LiNbO.sub.3 substrate and LiTaO.sub.3 substrate. However, in these known piezoelectric substrates, the electromechanical coupling coefficient K.sup.2 is small, the temperature coefficient of delay time TCD is not zero, the power flow angle PFA is not zero and a directionality inverted electrode structure could not be realized easily. Due to such problems, it is very difficult to realize an ideal surface acoustic wave device.
Furthermore, in order to obtain a surface acoustic wave device which is small in size, light in weight and low in power consumption, it is advantageous to make a propagating velocity of a surface acoustic wave lower. However, the above mentioned piezoelectric substrates have a relatively high velocity not lower than 3000 m/sec. Therefore, it is difficult to realize a small, light and low power surface acoustic wave device by using the known piezoelectric substrates.